The present invention relates to a silent chain power transmitting apparatus for transmitting power between sprockets with different numbers of teeth as achieved when driving camshafts or a balancer shaft from a crankshaft of a four-stroke engine.
In a four-stroke engine shown in FIG. 10 hereof, a silent chain power transmitting apparatus used for driving intake valves, exhaust valves and a balancer from a crankshaft has a valve driving crankshaft sprocket 11a, an intake valve driving camshaft sprocket 12a, an exhaust valve driving camshaft sprocket 12b, a balancer driving crankshaft sprocket 11b, a balancer shaft sprocket 13, a camshaft driving silent chain 20a, a balancer shaft driving silent chain 20b, a tensioner device composed of a tensioner lever L and a tensioner T cooperative to apply an appropriate tension on the silent chain 20, and a chain guide G for guiding the silent chain 20a against oscillation.
The intake and exhaust valves are designed to complete a single cycle of operation while the crankshaft makes two revolutions. Thus, the number of teeth of the camshaft sprockets 12a, 12b is twice the number of teeth of the camshaft driving crankshaft sprocket 11a. The sprockets 11a. 12a, 12B and the silent chain 20a form a speed decreasing power transmitting mechanism.
The balancer is designed to cancel out high-order unbalance by completing two revolutions while the crankshaft makes a single turn or revolution. To this end, the number of teeth of the balancer shaft sprocket 13 is half the number of teeth of the balancer driving crankshaft sprocket 11b. The sprockets 11b, 13 and the silent chain 20b form a speed increasing power transmitting mechanism.
The camshaft driving silent chain 20a and the balancer shaft driving silent chain 20b are substantially identical in construction, and though not shown in detail, they are constructed such that when the silent chain 20a, 20b is stretched straight under tension, respective central swelled portions of the inside flanks of a pair of link teeth of each link plate project outward from rectilinear outside flanks of the link teeth of the preceding link plate. With this construction, when the link plates of the silent chain 20a, 20b start to mesh with teeth of the sprocket 11a, 11b, 12a, 12b, 13, the inside flanks of the link plates come into contact with the sprocket teeth. This engagement is taken over by subsequent engagement of the outside flanks with the sprocket teeth, as achieved when the link plates of the silent chain are wrapped around the sprocket. When the link plates are placed in full meshing engagement with the sprocket teeth, the outside flanks are completely seated on the sprocket teeth. By way of the meshing engagement between the outside flanks of the link plates and the sprocket teeth, the silent chain transmits power between the crankshaft and the camshafts or between the crankshaft and the balancer shaft.
The crankshaft sprockets 11a, 11b, the camshaft sprockets 12a, 12b and the balancer shaft sprocket 13 are formed by a sprocket tooth hobbing process. In the hobbing process, a hob cutter with teeth profiled similarly with the profile of a series of trapezoidal teeth formed by the rectilinear outside flanks of the longitudinally overlapping link plates described previously is used to cut the periphery of a cylindrical workpiece with an amount of addendum modification set in such a manner that the silent chain 20a, 20b, as it is wrapped around the sprocket, forms an equilateral polygon with one side equal in length to a pitch of the silent chain. Thus, an involute tooth form is produced.
Operation of the conventional silent chain power transmitting apparatus of the foregoing construction will be described in conjunction with one of the sprockets 11a, 11b, 12a, 12b and 13 with different numbers of teeth.
In the conventional silent chain power transmitting apparatus, the silent chain 20a, 20b, as it is wrapped around the sprocket, forms an equilateral polygon while the outside flanks of each pair of link teeth are being seated on tooth flanks or surfaces of alternate sprocket teeth. In this instance, since the central swelled portion of the inside flanks of one link plate projects outward from the rectilinear outside flank of the preceding link plate, the inside flank of the link plate come into contact with the sprocket tooth flank and temporarily raises or lifts up the silent chain before the outside flank is seated on the same sprocket tooth flank Such temporary chain lifting seeks to suppress the equilateral polygonal chain motion, generally called xe2x80x9cchordal actionxe2x80x9d of the silent chain, which involves up and down movement or oscillation of the chain in a radial direction of the sprocket when the chain starts to mesh with the sprocket.
However, due to a great difference in the number of teeth between the crankshaft sprocket 11a and the camshaft sprockets 12a, 12b, or between the crankshaft sprocket 11b and the balancer shaft sprocket 13, the angles of rotation of the sprockets 11a, 11b; 12a, 12b and 13, which cause the silent chain to advance by one chain pitch, are different from one another. Consequently, the amplitudes of the chordal actions occurring at the respective sprockets vary to a considerable extent.
In the case where a silent chain having link teeth formed with centrally swelled inside flanks is used, the silent chain approaching a sprocket is lifted up by a constant amount of distance. Accordingly, the chordal action with respect to a selected one of the sprockets 11a, 11b, 12a, 12b and 13 can be suppressed. However, it is extremely difficult to suppress all of the chordal actions associated with these sprockets 11a, 11b, 12a, 12b and 13.
Furthermore, even when the chordal action occurring with respect to one of the 11a, 11b, 12a, 12b and 13 is suppressed, it is not considered from the viewpoint of the silent chain power transmitting apparatus as a whole that vibration and noises resulting from the chordal action are effectively reduced. Rather, when the central swelled portions of the inside flanks impinge against the sprocket tooth flanks, a beating motion of the silent chain occurs, producing additional sorts of vibration and noise and fluctuating the chain tension.
It is therefore an object of the present invention to provide a silent chain power transmitting apparatus for transmitting power between sprockets with different numbers of teeth, as achieved when camshafts or a balancer shaft is driven by a crankshaft of a four-stroke engine via a silent chain, in which the chordal action of the silent chain is completely suppressed with respect to all of the sprockets to thereby reduce vibration and noises caused the chordal action, and the changes in the chain tension is suppressed to thereby improve the driving performance and the durability of the silent chain power transmitting apparatus.
To achieve the foregoing object, according to the present invention, there is provided a silent chain power transmitting apparatus comprising; an endless silent chain composed of a multiplicity of link plates interleaved and articulately connected together by connecting pins, each link plate having a pair of generally V-shaped link teeth and a pair of pin holes for fitting the connecting pins, the V-shaped link teeth having inside and outside flanks; and at least two sprockets spaced from each other and having different numbers of teeth for mesh with the V-shaped link teeth of the link plates. When the silent chain starts to mesh with each of the sprockets, the inside flank of one link plate comes in contact with a tooth of the sprocket, and as the silent chain is wrapped around and subsequently placed in full meshing with the sprocket, the outside flank of the preceding link plate comes in contact with and subsequently becomes completely seated on the same teeth of the sprocket, thereby ensuring that the silent chain transmits power between the sprockets. The sprockets each have a tooth form formed by hobbing with a hob cutter having teeth profiled similarly with the profile of a series of trapezoidal teeth formed by the inside flanks of the longitudinally overlapping link plates described previously to cut the periphery of a cylindrical workpiece with an amount of addendum modification set in such a manner that an engagement height of the inside flank at the onset of the meshing engagement between the silent chain and the sprocket is in equal to a center distance between the center of the sprocket and the center of each connecting pin when the silent chain is wrapped around and seated on the sprocket. The outside flanks of the silent chain have a first portion profiled by an envelope that is tangent to every one of a group of tooth forms of the sprockets when the silent chain is wrapped around the sprockets with different numbers of teeth.
In one preferred form of the present invention, the first outer flank portion of each link tooth is disposed remotely from the tip of the link tooth, and the outside flanks further include a rectilinearly profiled second portion disposed adjacent to the tip end of each link tooth. The inside flanks of each link tooth have a rectilinear profile, and the outside flank of one link plate is retracted inward from the rectilinearly linearly profiled inside flank of the succeeding link plate.
The outside flanks may be arcuately concaved at the junction between the first outside flank portion and the rectilinearly profiled second portion.
In operation of the silent chain power transmission apparatus, the inside flank of one link plate comes in contact with a tooth of the sprocket at the onset of the meshing engagement between the silent chain and the sprocket. Then, as the silent chain is wrapped around and subsequently placed in full meshing with the sprocket, the outside flank of the preceding link plate comes into contact with and subsequently becomes completely seated on the same sprocket tooth, thereby ensuring that the silent chain transmits power between the sprockets.
By virtue of the two-stage or time-phased engagement, a meshing sound generated during meshing engagement between the silent chain and the sprocket is born separately by the inside flank and the outside flank. It is also possible to suppress stress concentration, which may occur at a crotch portion between the V-shaped link teeth when the silent chain is wrapped around the sprocket.
In the silent chain power transmitting apparatus of the present invention, the sprockets with different numbers of teeth have a tooth form formed by hobbing with a hob cutter having teeth profiled similarly with the profile of a series of trapezoidal teeth formed by the inside flanks of the longitudinally overlapping link plates described previously to cut the periphery of a cylindrical workpiece with an amount of addendum modification set in such a manner that an engagement height of the inside flank at the onset of the meshing engagement between the silent chain and the sprocket is in equal to a center distance between the center of the sprocket and the center of each connecting pin when the silent chain is wrapped around the sprocket. By thus profiling the sprocket teeth, the inside flanks of the link plates approaching the sprocket in the tangential direction always have an engagement height which is correspondent with the apex of an equilateral polygonal chain motion. This arrangement ensures that the silent chain and the sprocket assume a rack-and-pinion mechanism and can perform a smooth meshing operation free of fluctuations of the meshing speed. Furthermore, the equilateral polygonal chain motion, i.e., the chordal action, which occurs in the conventional silent chain as it approaches the sprocket, can be avoided.
In addition, the outside flanks of the silent chain have a tooth form profiled by an envelope that is tangent to every one of the tooth forms of the sprockets when the silent chain is wrapped around the sprocket By thus profiling the sprocket tooth form, the center distance between the sprocket center and the connecting pin centers of those link plates which are seated on the sprocket teeth along the outside flanks becomes equal to the engagement height of the link plate approaching the sprocket, regardless of the numbers of teeth of individual sprockets. This ensures that the silent chain becomes seated on the sprocket without producing a beating motion.
It will be appreciated form the foregoing description that with respect to any one of the sprockets with different numbers of teeth, from the start of meshing engagement between the silent chain and the sprocket to the end of seating of the silent chain on the sprocket, the distance between the center of the sprocket and the chain pitch line of the silent chain approaching the sprocket in the tangential direction is kept always constant. This ensures that both the chordal action of the chain and the beating motion of the chain can be suppressed.
The silent chain power transmitting apparatus of the present invention is also possible to suppress both the chordal chain action and the beating chain motion when used with two or more sprockets with the same number of teeth.
The term xe2x80x9cat the onset of the meshing engagementxe2x80x9d used herein represents a time point at which the inside flank of one link plate of the silent chain starts to contact a tooth of the sprocket from a tangential direction of the sprocket. The term xe2x80x9cwhen the silent chain is wrapped around the sprocketxe2x80x9d used herein represents a time point at which after the onset of the meshing engagement, the inside flank has loft the tooth of the sprocket, and the outside flank or the preceding link plate of the silent chain is seated on the same sprocket tooth while the silent chain is wrapped around the sprocket.
The term xe2x80x9cengagement height of the inside flankxe2x80x9d used herein represents a distance between a chain pitch line of one link plate approaching the sprocket at the onset of the meshing engagement and the center of the sprocket. The term xe2x80x9cchain pitch linexe2x80x9d used herein represents a line that interconnects the centers of connecting pins inserted through one link plate.
The term xe2x80x9cseated height of the outside flankxe2x80x9d used herein represents a distance between the chain pitch line and the center of the sprocket when the chain is wrapped around the sprocket in a manner to assume an equilateral polygon.
The term xe2x80x9csprockets having different numbers of teethxe2x80x9d is used herein in a comprehensive sense, i.e., to broadly refer to two or more sprockets each having a different number of teeth. There are no limitations about the numbers of teeth of the sprocket used provided that the sprockets can mesh with a silent chain used in combination therewith. To ensure that the outside flanks of the silent chains are stably seated on the sprocket teeth, the sprockets preferably have more than eighteen (18) teeth.
The xe2x80x9clink platesxe2x80x9d connected together to form an endless silent chain all have an identical surface profile with respect to each of the inside flanks and the outside flanks. The distance between the inside flank of one link plate and the corresponding inside flank of the preceding link plate is equal to a chain pitch P which is defined as a center distance between the pair of the connecting pins inserted through one link plate.
At the onset of the meshing engagement, the silent chain the sprocket assume a well known meshing state established between a rack and a pinion. Accordingly, the xe2x80x9cinside flanksxe2x80x9d of the generally V-shaped link teeth preferably have a generally rectilinear profile that can form a series of trapezoidal teeth of a rack.
The xe2x80x9coutside flanksxe2x80x9d of the generally V-shaped link teeth have, at least at a part thereof, a shape profiled by an envelope that is tangent to every one of the tooth forms of the sprockets when the silent chain is wrapped around the sprocket. The shape of the outside flanks is independent from the overall shape of the V-shaped link teeth. The outside flanks preferably have a rectilinear profile at a portion adjacent to a tip end of the corresponding V-shaped link tooth, the rectilinear profile having the same pressure angle as the inside flanks and being tangent to the tooth form of one sprocket having a minimum number of teeth. By thus profiling the outside flanks, it is possible to improve the durability of the V-shaped link teeth
When the silent chain is stretched substantially straight under tension, the outside flanks of one link plates, which are profiled by the envelope that is tangent to every one of the tooth forms of the sprocket, are retracted from the substantially rectilinearly profiled inside flanks of the adjacent link plates.